1. Field of the Invention
This invention relates to a power system that converts heat energy into motive work.
2. Background of the Invention
The Industrial Revolution has been fueled with petrocarbons such as coal, oil and natural gas. From the time of earliest records to the middle 1600's, human population grew at a very slow rate. Since about 1650, startling increases in human population have closely followed the exploitation of resources such as petrocarbons, metallic ores, water, and air. The fossil equivalent of some 180 million barrels of oil are burned each day by earth's human population. At the beginning of the 21st century, earth's population will reach six billion persons which doubles the 1960 population.
For millions of years, fossil deposits provided safe and natural storage of carbon and radioactive elements. Global combustion of 2,800 million tons of coal each year releases about 10,200 tons of carbon dioxide, 8,960 tons of thorium and 3,640 tons of uranium to the air, water, and food chain.
Burning the fossil equivalent of 180 million barrels of oil per day has polluted the global atmosphere with carbon dioxide and other objectionable emissions. The present concentration of carbon dioxide in the atmosphere is about 25 to 30% greater than at any time in the last 160,000 years. This increased presence of carbon dioxide traps solar energy in the atmosphere. Because more energy is trapped in the atmosphere, there is more evaporation of the oceans. This results in more extreme weather-related events such as floods, hurricanes, and tornados. Combustion of fossil fuels for generation of electricity exceeds all other sources of carbon dioxide pollution by the machines of the Industrial Revolution.
The market for electricity exceeds seven hundred billion dollars annually and is expected to reach one trillion dollars early in the 21st century. Generation of electricity to meet this demand must utilize renewable resources in order to prevent catastrophic degradation of the environment by fossil fuel combustion. Solar energy provides a vast but relatively untapped source of dependable energy. There have been developed various alternate fuel power systems that convert heat, wind, solar energy, etc. into electrical power. FIG. 1 shows a solar conversion system of the prior art. The system includes a solar reflector 2 which has a plurality of mirrors 4 that reflect sunlight to a focal point. Located at the focal point is a receiver 6 which is heated by concentrated solar energy. The heat is transferred to a working fluid that flows into a heat engine such as a Stirling engine 8. The Stirling engine 8 converts the thermal energy into mechanical energy. This mechanical energy is converted into electrical power by a suitable generator 9 that is coupled to the engine 8. The working fluid flows through a heat exchanger that is cooled by a suitable fluid of a cooling system. The cooling fluid flows through a radiator heat exchanger that is cooled by air that is delivered to the heat exchanger by a fan.
The electrical power generated by this type of conversion system is typically provided to a municipal power line, an off grid application such as a remote town, or a single farm or family unit. To be commercially competitive, the cost per unit energy produced by the system must be comparable to conventional power systems. The cost per unit of energy is a direct function of the energy efficiency of the system. In general, the system will have a lower cost per energy unit with a higher energy efficiency. Therefore it is desirable to maximize the net energy efficiency of the conversion system which is generated energy less parmetrics energy.
The efficiency of a heat engine such as a Stirling engine is generally defined by the equation B(Th-Tc)/Th, where Th is the upper working temperature, Tc is the lower working temperature and B is an efficiency factor of the engine that takes into account other losses. As shown by the equation, the efficiency can be increased by decreasing the lower temperature of the fluid. This is typically accomplished by increasing the heat transfer rate of the cooling system. The lower fluid temperature is limited by the ambient air temperature and amount of power drawn by the fan to cool the working fluid. At some point more energy is consumed cooling the working fluid than is gained by the system. For this reason, solar conversion systems are typically designed to operate at optimum pre-set cooling fluid temperatures.
To conserve energy, the radiator fan is de-energized when the cooling fluid is below the set point temperature. The fan is then energized when the fluid temperature raises to the set point temperature. The cycle of energizing and de-energizing the fan continues throughout the operation of the engine. The constant on-off cycling of the fan has numerous negative ramifications on the system. For example, energizing the fan initially requires a relatively large starting current which reduces the overall efficiency of the system. Periodic current surges also reduce the life of the fan. It would be desirable to provide a solar conversion system that minimized the power surges to the fan.
The system may have different optimum operating temperatures depending upon the temperature of the surrounding atmosphere and available solar energy. For example, the system may have one optimum lower fluid temperature when the ambient temperature is at 20.degree. C. and a different optimum lower fluid temperature when the ambient temperature is at 40.degree. C. Therefore a conversion system that has an optimum set point temperature for an ambient temperature of 40.degree. C. may not operate at maximum efficiency when the ambient temperature is 20.degree. C.
It would be highly desirable to have a solar energy conversion system which operates at maximum efficiency in varying ambient conditions.